1. Field of Invention
The present invention is directed to a process for producing biodiesel from crude tall oil (CTO). More specifically, the present invention is directed to a process for producing biodiesel by acid catalyzed reaction with alcohol, or by acyl halide reaction of CTO. This alternate reaction step is followed by separation and purification of the biodiesel product by solvent extraction and vacuum distillation.
2. Background of the Prior Art
Biodiesel, which is a term encompassing lower alkyl esters of fatty acids, especially methyl and ethyl esters of fatty acids, is currently the subject of much attention because it is a clean, renewable and biodegradable replacement fuel for conventional diesel fuel. This interest is based on the capability of biodiesel to serve as a complete or partial, e.g. blending with conventional diesel fuels, replacement of diesel fuel.
At present, production of biodiesel is limited to that obtained from the processing of vegetable oils and animal fats in which triglycerides found in those materials are transesterified, by reaction with methanol or ethanol in the presence of a base catalyst, such as sodium or potassium hydroxide, an acid catalyst, such as sulfuric acid, or an enzyme, such as lipase. These processes of producing biodiesel provide an additional benefit, it also yields glycerin, a commercially important compound, as a by-product.
Biodiesel, produced by one of these prior art processes, yields inadequate volume given its effectiveness as a diesel engine fuel. The combustion of biodiesel results in significantly lower emissions of carbon dioxide, particulates and unburned hydrocarbons compared to the emissions resulting from the combustion of conventional diesel fuel. As such, the use of biodiesel provides improved air quality results. Moreover, biodiesel, being a potentially carbon-neutral fuel, aids in amelioration of the greenhouse effect.
Crude tall oil (CTO) is generated during the kraft pulping process. During this process reacted resins obtained from wood are decanted from black liquor before an evaporation process. The decant is denoted as crude tall oil soap. This tall oil soap is decanted from spent cooking liquor in a soap separator. The soap is treated, such as by reaction with carbon dioxide, to produce CTO.
The annual global production of tall oil is about 1.6 million metric tons. Indeed, tall oil supplies about two thirds of the fatty acids used in the United States for industrial purposes. This large production of tall oil is not readably marketable due to weak demand for the prior art products produced from CTO. As a result, there has been a increasing inventory build-up of CTO insofar as the specialty products, into which CTO is presently converted by chemical companies, cannot economically be transmitted to chemical processing plants which are usually located at considerable distances from kraft pulping mills. As such, CTO, which is very viscous, malodorous and sticky, finds itself to be a low value product. This is magnified by the availability of alternative raw materials to produce rosins and varnishes, the products into which CTO was principally converted in the past.
The above remarks emphasize the confluence of a strong need in the art for both a new source of biodiesel and a new utility for CTO.
As stated above, biodiesel is presently produced from vegetable oil (VO), waste vegetable oil (WVO) and fats. Biodiesel is presently produced from any one of these products either by base or direct acid catalyzed transesterification with an alcohol or by conversion to fatty acids and then to alkyl esters with an acid catalyst. Of these methods, base catalyzed transesterification is most common.
Ma et al., Bioresource Tech., 70, 1 (1999) report that the transesterification of glycerides in VO/WVO with an alcohol to produce biodiesel is affected by the glycerides to alcohol molar ratio, reaction time and temperature and the free fatty acid and water content of the vegetable oil or fat.
The prior art includes a series of studies wherein such vegetable oils as soybean oil, sunflower oil, rapeseed oil, palm oil and WVO have been successfully employed in the production of biodiesel.
Additional studies have been conducted to characterize properties of biodiesel and engine emission tests utilizing biodiesel fuel. The conclusions drawn from these studies are that biodiesel serves as a very good substitute for conventional diesel fuel. Indeed, biodiesel is used in the same applications as diesel oil and has been found to have fuel properties similar to or even superior to traditional diesel oil. However, biodiesel provides the added benefits that its source is renewable and that it produces significantly lower emissions of pollutants.
There is very little prior art directed to the production of biodiesel from CTO. Liu et al., Petrol Sci. & Tech., 16 (5-6), 597 (1998) developed a process for producing a diesel oil additive, not biodiesel, from pine oil. That process produced a diesel oil cut which was blended with a base diesel fuel. That blended fuel was subsequently used in road and emissions tests.
The web site of Canadian Renewable Fuels Assn. mentions hydrotreating as a means of converting CTO into biofuels and fuel additives. However, this process is a hydrogenation process which produces hydrocarbon products rather than biodiesel.
Coll et al., Energy & Fuels, 15, 1166 (2001) describe the conversion of the rosin acid fraction of CTO by means of catalytic hydrotreatment into a diesel fuel additive. This process requires high hydrogen pressures of between 100 to 150 bars and elevated temperature of between 350° C. and 400° C. As such, this process is suitable for sophisticated oil refineries only. Although the process of Coll et al. does not employ an esterification process, it is believed to be the only prior art process that suggests the possibility of esterifying the rosin acid fraction of CTO. However, no specifics for such a process are provided.